This disclosure relates to a system and a method for the production of hydrogen. In particular, this disclosure relates to a system and a method for the production of hydrogen using a solid oxide electrolyzer in conjunction with a high temperature heat source.
Fossil fuel combustion has been identified as a significant contributor to numerous adverse environmental effects. For example, poor local air quality, regional acidification of rainfall that extends into lakes and rivers, and a global increase in atmospheric concentrations of greenhouse gases (GHG), have all been associated with the combustion of fossil fuels. In particular, increased concentrations of GHG's are a significant concern since the increased concentrations may cause a change in global temperature, thereby potentially contributing to global climatic disruption. Further, GHG's may remain in the earth's atmosphere for up to several hundred years.
One problem associated with the use of fossil fuel is that the consumption of fossil fuel correlates closely with economic and population growth. Therefore, as economies and populations continue to increase worldwide, substantial increases in the concentration of GHG's in the atmosphere are expected. A further problem associated with the use of fossil fuels is related to the inequitable geographical distribution of global petroleum resources. In particular, many industrialized economies are deficient in domestic supplies of petroleum, which forces these economies to import steadily increasing quantities of crude oil in order to meet the domestic demand for petroleum derived fuels.
Electrolyzers are an approach for producing hydrogen either at large central facilities or distributed at the point of use. An electrolyzer uses electricity to separate or split water into its components—hydrogen and oxygen. Today, two types of electrolyzers are used for the commercial production of high-purity hydrogen—alkaline and proton exchange membrane (PEM). But these approaches cannot currently compete, on an economic basis, with hydrogen produced by steam methane reforming (SMR) of natural gas.
However, SMR is highly dependent on the price and availability of natural gas. SMR also produces large amounts of carbon dioxide (generally about 12 kilograms of carbon dioxide equivalent per kg of hydrogen produced).
Systems have been proposed that couple a solid oxide electrolysis system to a helium-cooled nuclear reactor heat source using a steam Rankine cycle. High pressure steam is generated in a boiler heated by a primary loop that uses helium as a coolant. The high pressure steam is partially expanded through a steam turbine to produce electrical energy. A portion of the partially expanded steam is then reheated through a second heat exchanger heated by the helium from the primary loop. This intermediate pressure reheated steam can then be used for applications such as solid oxide electrolysis. This type of system risks steam ingress into the nuclear core due to the high-pressure steam generators, where the steam can be at a higher pressure than the primary helium coolant. Steam ingress into the core is undesirable because it can corrode the graphite moderator and graphite-coated fuel, and can also cause a reactivity insertion due to the moderating effect of steam. A further shortcoming of these systems is that the electrical generation and hydrogen generation are coupled together in the same system and are in fluid communication with each other, making the system inflexible and potentially not optimized.
Solid oxide electrolysis systems have been proposed that do not comprise an air compressor operative to sweep oxygen produced in the anode of the cell out of the cell. Instead, these systems allow the oxygen produced to accumulate in the anode until a sufficient oxygen pressure is achieved to collect and store this high-purity oxygen at pressure. These systems will require additional electrical energy to drive the electrolysis of steam into hydrogen and oxygen because of the high oxygen partial pressure on the anode side of the cell. Additionally, these systems may be limited to low current densities and therefore low hydrogen production per unit area of cell because these systems do not have a sweep gas to remove waste heat from the anode.
Solid oxide electrolysis systems have been proposed that comprise an air compressor operative to sweep oxygen produced in the anode of the cell out of the cell, and further comprise a heat exchanger to preheat this air prior to injection into the anode by transferring heat from the helium exiting the nuclear reactor core. These systems thus require an additional heat exchanger that interfaces with the nuclear reactor, which incurs additional cost and introduces a risk of air ingress into the nuclear reactor.
Solid oxide electrolysis systems have been proposed that utilize steam to sweep the oxygen produced at the anode side of the cell and further to remove waste heat produced at the anode side of the cell. These systems may suffer corrosion or loss of performance of the anode due to the presence of steam at the anode.
In order for electrolyzer systems to be commercially viable, reduced capital cost and increased system efficiency are desirable. It is therefore desirable to use high-temperature solid oxide electrolyzers that can make use of a high-temperature heat source, such as helium-cooled nuclear reactor to reduce the amount of electrical energy required to drive the electrolysis process.